Displays utilizing two dimensional arrays, or matrices, of pixels each containing one or more light emitting devices are very popular in the electronic field and especially in portable electronic and communication devices, because large amounts of data and pictures can be transmitted very rapidly and to virtually any location.
Light emitting diode (LED) arrays are becoming more popular as an image source in both direct view and virtual image displays. One reason for this is the fact that LEDs are capable of generating relatively high amounts of light (high luminance), which means that displays incorporating LED arrays can be used in a greater variety of ambient conditions. For example, reflective LCDs can only be used in high ambient light conditions because they derive their light from the ambient light, i.e. the ambient light is reflected by the LCDs. Some transflective LCDs are designed to operate in a transmissive mode and incorporate a backlighting arrangement for use when ambient light is insufficient. In addition, transflective displays have a certain visual aspect and some users prefer a bright emissive display. However, these types of displays are generally too large for practical use in very small devices.
Also, organic light emitting diodes or organic electroluminescent devices (OLED or OED, hereinafter OED) and arrays thereof are emerging as a potentially viable design choice for use in small products, especially small portable electronic devices, such as pagers, cellular and portable telephones, two-way radios, data banks, etc. OED arrays are capable of generating sufficient light for use in displays under a variety of ambient light conditions (from little or no ambient light to bright ambient light). Further, OEDs can be fabricated relatively cheaply and in a variety of sizes from very small (less than a tenth millimeter in diameter) to relatively large (greater than an inch) so that OED arrays can be fabricated in a variety of sizes. Also, OEDs have the added advantage that their emissive operation provides a very wide viewing angle.
Generally, OEDs include a first electrically conductive layer (or first contact), an electron transporting and emission layer, a hole transporting layer and a second electrically conductive layer (or second contact). Generally, the various layers must be positioned on a planar surface at least defining the extent of the OED. The light can be transmitted either way but must exit through one of the conductive layers. There are many ways to modify one of the conductive layers for the emission of light therethrough but it has been found generally that the most efficient LED includes one conductive layer which is transparent to the light being emitted. Also, one of the most widely used conductive, transparent materials is indium-tin-oxide (ITO), which is generally deposited in a layer on a transparent substrate such as a glass plate.
A major problem with OEDs utilizing a conductive, transparent layer is the high resistivity of the conductive transparent material. ITO, for example, has a resistivity of approximately 50 ohms/square. Further exacerbating this problem is the fact that OEDs are current driven devices (i.e. emit due to current flowing through them), as opposed to voltage driven devices, such as LCDs. Thus, the high resistivity contact of the OED becomes virtually prohibitive when attempting to place OEDs in large arrays.
Another problem complicating the fabrication of OED arrays is the fact that the various layers of each OED must be deposited on a planar surface, thus, complicating the integration of an active network into the array. If an active circuit for each pixel of the array is positioned adjacent and to one side of each pixel, for example, the fill factor (the ratio of luminescent area to non luminescent area) for the array will be so poor it will produce unsatisfactory images.
Accordingly, it would be beneficial to provide an active OED array and driving apparatus which overcomes these problems.
It is a purpose of the present invention to provide a new and improved active OED array and driving apparatus.
It is another purpose of the present invention to provide a new and improved active OED array and driving apparatus with a high fill factor.
It is still another purpose of the present invention to provide a new and improved active OED array and driving apparatus which is easier and less expensive to fabricate and use.
It is a further purpose of the present invention to provide a new and improved active OED array and driving apparatus in which the OED, active circuits and drivers are integrated onto a single substrate.
It is a still further purpose of the present invention to provide a new and improved active OED array and driving apparatus in which the OED, active circuits and drivers are integrated onto a single substrate and passivated to improve reliability.